Conventional plastic impellers are constructed in two parts, often due to the limitations of injection molding and the specific geometries required. As shown in FIGS. 1-4, a conventional plastic impeller 10 includes a first piece 12 with impeller vanes 14, a back plate 16, and a motor mounting feature 18 (e.g., a motor hub) integrally molded together. The conventional plastic impeller 10 also includes a second piece 20 (e.g., a cover or a shroud) including an inlet nose 22 and an eye 24.
Conventional fabrication processes require a minimum of two secondary operations to form a complete impeller 10. First, the first piece 12 and the second piece 20 are mechanically bonded together. Second, the nose 22 must be machined to be concentric to the hub 18 (e.g., to a specified value A, as shown in FIG. 3). Conventional bonding processes, such as ultrasonic, vibration, hotplate adhesives, etc., use part-holding fixtures. As a result, these processes require clearances in the fixtures and their mating impeller parts, as well as clearances associated with aligning the fixtures relative to each other, in order to maintain concentricity between the hub 18 and the nose 22. Bonding processes that involve vibration and/or part movement introduce additional issues with regard to maintaining concentricity. General wear from use of the fixtures further impairs the concentric relationship between the hub 18 and the nose 22. The resulting concentricity issues are corrected by machining additional clearances into the fit between the nose 22 and a wear ring 26 of a diffuser 28, as shown in FIG. 4 (e.g., by changing value B in FIG. 3). When the impeller 10 is rotated by an electric motor at relatively high speeds, these additional clearances provide room for vibration. This can result in potential bearing damage, as well as unwanted noisy operation. In addition, these clearances provide room for internal leakage during the pumping process, as shown in FIG. 4, which reduces the mechanical efficiency of the pump.